In WDM (Wavelength Division Multiplexing) networks, fabricating light sources of pure spectral content and multiplexers which are high-performance and low cost are essential for developing the fiber networks. When applying in the WDM network, the distributed feedback (DFB) lasers are the most popular single-wavelength lasers. In a DFB laser, some types of gratings have to be formed on the longitudinal structure thereof for forming a wavelength selection so as to fabricate the single-wavelength laser. This kind of laser has excellent stability and reliability and has been extensively applied in high-performance fiber transmission systems.
Generally, this kind of laser is requested to have a high SMSR (Side-Mode Suppression ratio). However, to be limited by the specific physical characteristic itself, measures have to be taken for achieving high SMSR. Therefore, the yield of fabricating single-wavelength DFB lasers is generally low and this increases the cost. Thus, if we want to employ this kind of lasers in a low price fiber system (such as Metro or Access network), this problem has to be solved firstly.
Currently, the manufacturing method of DFB lasers usually adopts an index-coupled structure, and two end-facets of the laser are respectively coated by an anti-reflection coating and a high-reflection coating for destroying the mode symmetry thereof so as to illuminate a single-wavelength light. However, the phases of the end-facets can significantly influence the output characteristic of the laser so that the position of the lasing wavelength cannot be controlled easily and the yield is low.
Although a utilization of loss-coupled and gratings can solve the yield problem of the single-wavelength laser described above, the shape of the grating has to be precisely controlled, otherwise, will cause a high threshold current. Another method also for solving the problem described above is the utilization of gain-coupled gratings. However, it needs to etch into the gain material, or a complicated epitaxy technique must be employed, so the reliability has to be concerned.
Thus, the present invention intends to solve the drawbacks in the prior arts described above so as to provide low price and high-performance single-wavelength lasers in WDM network.
As to a multi-wavelength laser array in the prior arts, the manufacturing methods therefor include:
(1) Altering the grating period of each laser through multiple holographic exposures;
(2) Altering the effective period of the grating through positioning the waveguide of each laser and the grating at a different angle;
(3) Forming the grating of each laser through electron-beam lithography so as to have different periods;
(4) Altering the effective refractive index of each waveguide through the selective area growth expitaxial growth;
(5) Altering the thickness of each waveguide through multi-etching technique so as to have different effective refractive index; and
(6) Altering the width of each waveguide on the mask so as to have different effective refractive index.
In method (1), it has to employ a specific holographic exposure and also will consume more time. Moreover, the performance of each laser in the laser array made from method (2) will be uneven from one to another, and the wavelength range that can be altered by method (2) is also limited. As to method (3), it will take a lot of time to fabricate the laser array so that it is not suitable for mass production. Furthermore, through method (4), it will not easy to fabricate a laser array having even wavelength intervals and accurate wavelength. Then, if the laser array is fabricated by method (5) or (6), the quality thereof will be easily influenced by the fabricating processes, and thus, the wavelength can not be easily controlled and the wavelength range that can be altered is also limited.
In addition, another conventional method of fabricating the multi-wavelength laser array is to employ a sampled grating DBR laser structure. However, it requires a longer length although it is easier than the methods described above and can provide an accurate wavelengths.
Consequently, because of the technical drawbacks described above, the applicant keeps on carving unflaggingly to develop a “structure and manufacturing method of single-wavelength and multi-wavelength distributed feedback lasers” through wholehearted experience and research.